1. Field of the Invention
The invention relates to an ultrasonic imaging system comprising an ultrasound endoscope device for scanning a patient's organs, in particular for use in transesophageal echocardiography.
2. Description of the Prior Art
Such a device specially adapted for transesophageal echographic scanning is for example disclosed in U.S. Pat. No. 5,105,819.
Transesophageal echocardiography (TEE) has become a widely used imaging technique for evaluating cardiac structure, function, and valvular anatomy. Transesophageal echocardiography has also provided a new perspective on the thoracic aorta, and there is growing evidence that the technique contributes valuable and sometimes unique information about aortic structure and pathology.
Two-dimensional (2D) transesophageal echocardiography (TEE) and 2D intravascular ultrasound (IVUS) imaging face their greatest limitation in visualizing aortic disease in patients. Recently introduced multi-plane transesophageal probes have improved visualization of the proximal and transverse aorta. Three-dimensional (3D) image reconstruction, TEE and IVUS can even improve further visualization but still provide only limited spatial appreciation in aortic disease because 3D imaging of the thoracic aorta requires a broader spatial visualization of the mediastinum than provided by both techniques. Another approach called 3D lighthouse transesophageal echocardiography (LTEE) uses a thin intravascular ultrasound catheter, which provides a full circumferential (360 degree) image, but is invasive and cannot be used during cardiac surgery with cardio-pulmonary bypass. Also these three methods (3D, IVUS, and LTEE) need special ultrasound equipment, which is not standard available.
The available evidence strongly supports the use of TEE in aortic dissection and atherosclerosis and suggests potential utility in additional diseases of the aorta such as aneurysm, ulceration, trauma, and congenital or inherited malformations.
The features of ultrasonic imaging systems cause main problems in visualization of certain organs in particular of the upper mediastinum, including the ascending aorta.
To understand these problems, it is important to know the physical limitations of ultrasound. Ultrasound consists of sound waves. The signal is determined by:    1. Frequency, f is determined by the generator.    2. Velocity, v is determined by the medium.    3. Wavelength, λ is the distance between two cycles of sound waves.
Absorption of sound waves is dependent of the medium. This is reflected as the half power distance: The distance in which half of the ultrasound energy will be absorbed. For water this is 360 cm, bone 10, 2 cm and for air 0, 06 cm. This means that nor bone nor air will not let through ultrasound waves in practice.
Consequently, the prior art does not achieve good imaging results, if air or bone are between the ultrasound source and the area which are to be investigated.